ULTRASTRUCTURE OF SEROTONIN-IMMUNOREACTIVE TERMINALS IN THE CORE AND SHELL OF THE RAT NUCLEUS-ACCUMBENS - CELLULAR SUBSTRATES FOR INTERACTIONS WITH CATECHOLAMINE AFFERENTS
Ej. Vanbockstaele et Vm. Pickel, ULTRASTRUCTURE OF SEROTONIN-IMMUNOREACTIVE TERMINALS IN THE CORE AND SHELL OF THE RAT NUCLEUS-ACCUMBENS - CELLULAR SUBSTRATES FOR INTERACTIONS WITH CATECHOLAMINE AFFERENTS, Journal of comparative neurology, 334(4), 1993, pp. 603-617
The nucleus accumbens is composed of a core region involved in motor f
unctions and a shell region implicated in emotional and motivational p
rocesses. Both of these regions receive serotonin- and dopamine-contai
ning afferents. We examined whether the serotonin innervation or relat
ion to catecholamine (mainly dopamine) axons in the nucleus accumbens
shows common features or specializations corresponding to the noted fu
nctional differences in core and shell subregions. To address this que
stion, we examined the ultrastructure of serotonin-containing axons an
d their relation to catecholamine-containing afferents in either the c
ore or shell of the nucleus accumbens. Single coronal sections through
the rat forebrain were processed for immunoperoxidase labelling of se
rotonin and immunogold silver labeling of tyrosine hydroxylase, the ca
techolamine-synthesizing enzyme. Varicose processes showing peroxidase
product for serotonin by light microscopy were confirmed to be axons
and terminals by electron microscopy. In a quantitative analysis of se
rotonin-immunoreactive terminals forming one or more contacts in singl
e sections, some common features were observed. For the core (n = 120)
and the shell (n = 82), 41% formed synaptic junctions with unlabeled
dendrites, 75% were in apposition with unlabeled terminals, which ofte
n formed asymmetric junctions, and 20% were in apposition with axons o
r terminals containing tyrosine hydroxylase. Thus, in both the core an
d shell of the nucleus accumbens, serotonin terminals synapse on posts
ynaptic neurons and are likely to modulate or be modulated by presynap
tic interactions with excitatory axons forming asymmetric junctions an
d by catecholaminergic afferents. Marked differences in the morphology
of serotonin axons were also seen in the core versus shell of the nuc
leus accumbens. By light microscopy, serotonin-immunoreactive axons we
re thicker and more varicose than those found in the core. Ultrastruct
ural analysis confirmed that, in contrast to the core, serotonin-immun
oreactive axons and terminals in the shell were larger in cross-sectio
nal diameter size (0.7 mum vs. 0.3 mum). Additionally, serotonin axon
terminals in the shell contained more numerous immunoreactive large de
nse core vesicles and more frequently formed symmetric as opposed to a
symmetric contacts with dendrites. The larger size and more numerous d
ense core vesicles in serotonin-immunoreactive terminals in the shell
support the concept that serotonin or co-existing neurotransmitter may
be more tonically released in the shell versus core of the nucleus ac
cumbens. (C) 1993 Wiley-Liss, Inc.